All that looks green is not grass. It can be moss too. Perfectly camouflaged from a distance, you have to get closer to be able to clearly distinguish it from your lawn. Rain, unforgiving soil and soggy lawns create a haven for moss.
Moss develops when there are compacted soils with poor drainage. A waterlogged lawn inhibits growth and encourages fungus to develop that can cause grass thinning over time, and finally killing it. This creates an ideal environment for moss and weed to grow.
Excessive shade from tall trees or building structures is another reason for thinning lawns. Even most shade tolerant grasses require a minimum of four hours of direct sunlight and in its absence the blades reach out for sunlight, becoming weak and undernourished.
Soil compaction and alkaline soils diminish the lawn’s composting capability, resulting in a dead, organic material (dried grass blades and thick-matted roots) accumulating above ground called thatch that blocks nutrient absorption to the roots. A dense thatch layer essentially chokes the grass and leads to water and fertilizer runoff, which in turn lead to shallower roots. Dethatching every couple of years lifts the dead material away, allowing breathing space for your grass and lets water, air and nutrients reach the soil.
Over time, soil naturally loses its nutrients by wind, rain, and soil erosion. Timely fertilization either with a store-bought product or compost is essential to replenish the lawn and help maintain its beauty. Avoid over-fertilization, as it leads to a faster growth that would not break down.
Other reasons for poor drainage could be the faulty grading (or lawn leveling) that results in water pooling. A properly-graded trench, or French drain, directs the flow of water away from the building structure. The trench is then filled in with gravel, which lets gravity do the work of channeling the water into a perforated pipe that sits at the trench’s base. Without proper grading, a French drain would not efficiently work.
But no matter what you do, it is going to be a constant battle. The wet Pacific Northwest weather is conducive to moss growth, and the short summers do not completely get rid of moss.
The annual budget required for keeping a moss-free lawn is sizable. For well-established older lawns, regular aeration is critical to resolve soil compaction and spring is an excellent time to do it before the dry summers harden the ground. A core aerator removes plugs of soil which not only loosens but also allows for air circulation to the roots.
Lawn mosses do not need much light or nutrition to proliferate. Moisture, however, is what gives it life. Thus, it is critical to focus eradication efforts in the fall, during its active growing period when the rains descend. Despite your best efforts, small patches of moss might show up. To eradicate them, spray diluted solutions of either vinegar or dish soap.
All said, the question remains—do we really need to remove moss?
My answer is no. Moss does no harm to the lawn and adds a natural beauty to the garden setting, typically filling in spaces of soil where little else will grow. It prevents soil erosion, is low maintenance, and does not require mowing. Japanese gardens use moss as an integral element of their horticultural art.
At the risk of sparking protests from lawn aficionados, I am all for embracing the notion of moss in your lawn and focusing your energy elsewhere.
Alternatively, if grass refuses to grow in a moss-prone area, you can try planting white clover, creeping thyme and other low growing groundcover perennials that would flourish and provide an aesthetically pleasing and consistent greenery.